1. Field of the Invention
The present invention relates to photoresist used during semiconductor processing. More particularly, the invention relates to methods for predicting photoresist profiles on semiconductor surfaces.
2. Description of Related Art
Predicting and accurately describing photoresist cross-sectional shapes after deposition of photoresist on semiconductor surfaces is an important metric for photoresist quality. Thin sections of photoresist may cause a short or break during subsequent semiconductor processing and lead to failure or decreased performance of a semiconductor device produced by the semiconductor processing. For example, during an etch process, a minimum thickness of photoresist is needed to properly transfer the photoresist pattern into the underlying surface (e.g., the semiconductor substrate).
Surface inhibition/enhancement models have been used to attempt to predict the behavior of photoresists and height loss during photoresist development with some measurable success. Current surface inhibition/enhancement models typically predict photoresist cross-sections with rounded top corners and a relatively flat top surface. FIG. 1 depicts a cross-sectional representation of an example of a photoresist profile found using current surface inhibition/enhancement models. Photoresist 102 is modelled as being formed on semiconductor wafer 100. As shown in FIG. 1, photoresist 102 has a cross-sectional profile with rounded top corners and relatively flat top surfaces. However, experimental cross-sectional data commonly shows photoresist profiles with completely rounded tops (e.g., dome-shaped profiles).
Additional modelling elements have been added to surface inhibition/enhancement models to attempt to more accurately predict the rounded top profile of photoresist. For example, acid contamination during PEB (post-exposure bake) has been added to produce relatively good cross-sectional shapes for photoresist profiles. Adding acid contamination includes adding acid to the top of the photoresist surface that was not created by the exposure process. This method, however, models a mechanism that is only likely when a top coat is used and rounded top profiles have been observed in photoresist systems that do not have top coats (e.g., immersion ArF photoresist systems). Thus, acid contamination during PEB is an unlikely mechanism for top loss in the photoresist. Additionally, top resist thickness loss is often relatively uniform across a variety of features and acid contamination does not predict uniform top loss.
Other photoresist models have included inhomogenous concentrations of other components of the photoresist (e.g., photoacid generator (PAG) or quencher components) to change the shape of the photoresist cross-section. For example, the concentration of the PAG or quencher component may have a gradient through the photoresist. Such models using PAG or quencher component inhomogenous concentrations, however, do not show resist loss in dark (unexposed) areas while resist loss has been observed experimentally in dark areas. Thus, there is a need for photoresist models that more accurately predict and simulate mechanisms that contribute to top loss during photoresist development (processing).